Article pubs.acs.org/JAFC
Ethanol Production from Food Waste at High Solids Content with Vacuum Recovery Technology Haibo Huang,† Nasib Qureshi,‡ Ming-Hsu Chen,† Wei Liu,† and Vijay Singh*,† †
Department of Agricultural and Biological Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ‡ Bioenergy Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service (ARS), United States Department of Agriculture (USDA), 1815 North University Street, Peoria, Illinois 61604, United States ABSTRACT: Ethanol production from food wastes does not only solve environmental issues but also provides renewable biofuels. This study investigated the feasibility of producing ethanol from food wastes at high solids content (35%, w/w). A vacuum recovery system was developed and applied to remove ethanol from fermentation broth to reduce yeast ethanol inhibition. A high concentration of ethanol (144 g/L) was produced by the conventional fermentation of food waste without a vacuum recovery system. When the vacuum recovery is applied to the fermentation process, the ethanol concentration in the fermentation broth was controlled below 100 g/L, thus reducing yeast ethanol inhibition. At the end of the conventional fermentation, the residual glucose in the fermentation broth was 5.7 g/L, indicating incomplete utilization of glucose, while the vacuum fermentation allowed for complete utilization of glucose. The ethanol yield for the vacuum fermentation was found to be 358 g/kg of food waste (dry basis), higher than that for the conventional fermentation at 327 g/kg of food waste (dry basis). KEYWORDS: food waste, ethanol, fermentation, vacuum recovery, high solids content
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gases (CH4 and CO2).15,16 Incineration is another way to manage food waste but is banned in different countries because of environmental concerns. Also, energy recovery through incineration may not be feasible, because of the energy loss to evaporate the large water content in food waste.16 Food waste can be diverted from landfills and incinerators by turning it into compost to improve the soil fertility, but it may cause severe pollution to surface and underground water.17 On the other hand, food waste contains abundant nutrition (starch, glucose, protein, etc.), making it a good raw material for biofuel production. Until now, most of the research has been focusing on the usage of food waste to produce biogas through anaerobic digestion.18−20 Food waste can also be used as a lowcost feedstock for producing ethanol,15,21,22 which is a more valuable fuel compared to biogas. Besides using the low-cost materials, fermentation at higher solids contents can lower the ethanol production costs, because it can reduce the energy and water consumptions as well as the volumes of the processing equipment.23 However, higher solids fermentation results in a higher ethanol concentration, which inhibits yeast activity, thereby causing reduced ethanol yield and fermentation efficiency.24 Vacuum stripping, one of the in situ ethanol removal technologies, has been reported to improve the ethanol or butanol fermentations at high solids contents.23,25−27 In a vacuum fermentation system, the produced ethanol is removed by maintaining the bioreactor under vacuum conditions, so that ethanol boils off at the fermentation temperature and is subsequently recovered by the
INTRODUCTION Over the past few decades, the objectives to establish national energy independence and to reduce the greenhouse gas emissions have led to the development of renewable biofuel technologies based on agricultural materials. Ethanol is by far the most significant biofuel in the United States, accounting for 94% of all biofuel production in 2012.1 Ethanol is mainly produced from corn in the U.S. and from sugar cane in Brazil.2 However, corn and sugar cane are also used as food; overuse of corn or sugar cane as feedstock for ethanol production would create “food versus fuel” competition. Furthermore, increasing prices of corn and sugar cane are the main drivers of the high cost of ethanol production. According to the previous studies, corn and sugar cane feedstock costs contributed to 70−90% of the total ethanol production costs.3−5 Researchers have investigated the production of ethanol from low-cost agricultural residues, such as corn stover,6,7 wheat straw,8 sugar cane bagasse,9 and rice straw.10 Efficiently releasing sugars from cellulose and hemicellulose is one of the main challenges of using cellulosic biomass.11 To break cellulose and hemicellulose into monosaccharides, the biomass materials have to be processed with a harsh pretreatment process, followed by hydrolysis with the addition of a high dosage of enzymes, which significantly increases the capital and processing costs of the ethanol production.11,12 Food waste is a complex biomass discharged from households, restaurants, cafeterias, and retail stores and accounts for a considerable portion of municipal solid waste.13 In the U.S., more than 36 million tons of food waste were generated in 2012 alone.14 Food waste management raises significant environmental concerns. Disposal of food waste in a landfill is not only costly but also causing potential environmental problems, with direct and indirect emissions of greenhouse © 2015 American Chemical Society
Received: Revised: Accepted: Published: 2760
November 14, 2014 February 19, 2015 February 23, 2015 February 23, 2015 DOI: 10.1021/jf5054029 J. Agric. Food Chem. 2015, 63, 2760−2766
Article
Journal of Agricultural and Food Chemistry
Figure 1. Experimental setup for the food waste fermentation with the vacuum stripping system. The arrows in the figure show flow direction. The vacuum fermentation system consisted of a 3 L water-jacked fermenter, a vacuum pump, a condensation system with a chiller and a coiled condenser, and a cold water trap. During the vacuum application, ethanol and water vapors evaporated from the fermentation broth and condensed in the condensation system. The escaped (uncondensed) ethanol and water vapors were collected in the cold-water trap. to the ethanol model solution for 1 h, during which the model solution boiled and the vapors were condensed in a 250 mL conical flask. At the completion of each vacuum application, the remaining solution in the fermenter, the condensate in the conical flask, and the solution in the cold water trap were collected and analyzed for volumes and ethanol concentrations. Experimental Materials and Reagents. The food waste was obtained from a local retail store in Urbana, IL, and mainly contained mashed potatoes, sweet corn, and white bread and was used as a model food waste. The moisture content of the food waste was 64.0%. The composition of the food waste (dry matter basis) was 63.5% starch, 4.3% glucose, 13.9% protein, 4.1% oil, 5.2% neutral detergent fiber, and 3.4% ash. The procedures of the composition measurements are provided in Analytical Procedures and Calculations. The high starch content in the food waste sample was very similar to the sample reported in the previous study.29 Received food waste was pulverized and mixed using a fruit/vegetable mixer for 3 min, analyzed for moisture content, and stored at 4 °C for the following experiments. For the fermentation slurry preparation, 10 N sulfuric acid (Ricca Chemical, Arlington, TX), solid urea (Fisher Scientific, Waltham, MA), and active dry yeast (Ethanol Red, Fermentis, Lesaffre Yeast, Milwaukee, WI) were used. Enzymes used in the fermentation processes were GHSE (Stargen 002) and protease (GC 212) obtained from Dupont Industrial Biosciences (Palo Alto, CA). Stargen 002 contained Aspergillus kawachi α-amylase expressed in Trichoderma reesei and a glucoamylase from Trichoderma reesei that work synergistically to hydrolyze granular starch substrate to glucose, and it had an activity of ≥570 GSHU/g (where GSHU = granular starch hydrolyzing units). Protease enzyme, GC 212, was obtained by fermentation of a selected strain of Aspergillus niger and was able to hydrolyze peptide bonds along a protein chain. It had an activity of 2000 SAPU/g (where SAPU = spectrophotometer acid protease units). Conventional Fermentation in a Batch Reactor. Fermentation slurry at 35% solid contents (w/w) was prepared by mixing 500 g of dry weight of mixed food waste with a calculated amount of deionized water. The fermentation slurry was then adjusted to pH 4.0 using 10 N sulfuric acid. Experiments were performed in a 3 L fermenter. The slurry was added with 3 mL of GSHE enzyme (Stargen 002), 0.2 mL of protease enzyme (GC 212), 2 mL of urea solution (50%, w/v), and 2 mL of prepared yeast culture. Yeast culture was prepared by dispersing 5 g of active dry yeast and 25 g of deionized water and agitated at 90 rpm at 32 °C for 20 min in an incubator shaker (C24 incubator shaker, New Brunswick, NJ). Fermentation was conducted for 72 h at 32 °C with continuous agitation at 30 rpm. The enzymatic
following condensation system with cooling or chilling water. In the vacuum fermentation system, a low ethanol concentration can be maintained in the fermenter during fermentation, thereby eliminating or minimizing yeast ethanol inhibition.23,27 The objective of this study was to produce ethanol from food waste at high solids content with vacuum recovery technology. Fermentation of food waste at high solids content results in a very viscous mash and a high glucose concentration (which also inhibits yeast). To overcome this problem, granular starch hydrolyzing enzyme (GSHE) was used to directly digest raw starch in food waste at low temperatures (
